TARGETED HCN1 CHANNEL BLOCK AS A PRECISION TREATMENT FOR <EM>HCN1 </EM>DEVELOPMENTAL AND EPILEPTIC ENCEPHALOPATHY

Pathogenic variants in the hyperpolarisation-activated, cyclic nucleotide-gated channel 1 (HCN1) gene can cause the HCN1 channel to lose voltage sensitivity and remain constitutively open. This allows cations to ‘leak’ through the channel, causing neuronal hyperexcitability. In patients, this results in a severe treatment-resistant developmental and epileptic encephalopathy (DEE). Blocking the cation ‘leak’ could be an effective treatment approach. However, broad-spectrum HCN channel inhibition causes significant side-effects. RO-275 is an HCN channel inhibitor with high selectivity for HCN1 over other HCN channel subtypes. We used a range of assays at single cell, network, and behavioural levels to explore the ability of RO-275 to normalise HCN1-DEE phenotypes. RO-275 significantly reduced hyperpolarisation-activated currents in HCN1^M305L DEE mutant channels expressed in Xenopus oocytes. When administered directly into the brains of mice harbouring the Hcn1^M294L DEE variant, RO-275 caused a significant reduction in epileptiform spiking without affecting underlying spectral activity. However, when administered systemically to Hcn1^M294L mice, RO-275 caused no significant change to epileptiform spiking, suggesting a limited ability to permeate the blood-brain barrier. RO-275 was also unable to rescue a hyperlocomotive phenotype in these mice. Together this data indicates that targeted HCN1 channel block may be an effective treatment approach for HCN1-DEE. Future efforts will be needed to develop compounds with enhanced blood-brain barrier permeability and to explore their ability to normalise both seizures and clinical comorbidities in HCN1-DEE.